It depends on the attitute of the arowana. Assuming your geo's have some decent size and you get a mild mannered arowana, it definately could work. I wouldnt try it with a Jardini though....maybe a silver.
Just keep in mind that they might eat anything that can fit in their mouths, but I've seen it work before.
My big silver wont really bother anything above 4 or 5" and he is about 24" long